1. Field of the Invention
This invention relates to a method for the activation of superconductors and to devices incorporating superconductors. It finds particular application in the fabrication of semiconductor devices which have integral regions of superconductive material.
2. Description of the Related Art
At present, there exist superconductors which exhibit the properties of a superconductor at temperatures substantially greater than 77.degree. K. (-196.degree. C.), the boiling point of liquid nitrogen. Such superconductors commonly comprise copper oxide ceramics typically fabricated from a mixture of yttrium oxide, barium carbonate and copper oxide. These constituent materials may simply be provided in a mortar in the volume ratios 1:2:3 and ground into a powder mixture. The mixture is then heated for about nine hours at temperatures in the region of 900.degree. C. to 1100.degree. C. At this temperature, the chemical reactions that produce the ceramic superconductor take place. It is then necessary to cool the ceramic at an accurately controlled rate so as not to degrade the superconducting properties.
The powder produced at this stage comprises superconducting material, the superconducting properties of which may be enhanced if the material is subject to a further process. This further process is included so as to `activate` the superconducting material by sintering or fusing the grains of the material and also, equilibrating the oxidation of the ceramic. Prior to this further process the ceramic powder is typically compressed so as to form a pellet of the material.
The further process typically includes heating the material for many hours at a temperature of up to 1000.degree. C. in an oxygen-rich environment, provided by introducing a flow of oxygen in the region of the material as it is heated. It is again important to control the rate at which the material cools after this further process, i.e. annealing, is complete.
The above described method of activating the superconductor is disadvantageous since it is unable to generate a substantial amount of superconductivity even with materials having a highly perfect crystal structure which is devoid of pores. This disadvantage is partly associated with the rate at which the oxygen diffuses through the material and also with the reactivity of the material even at high temperatures.
European Patent Application No. EP0286106A (Hitachi) describes a process for controlling an oxygen content of a non-superconductive or superconductive oxide in which a beam of particles such as ions, electrons, or neutrons or electromagnetic radiation is applied to the non-superconductive or superconductive oxide of a perovskite type such as YBa.sub.2 Cu.sub.3).sub.7-X, thereby increasing or reducing the oxygen content of the oxide at the sites of oxygen in the crystal lattice of the oxide. Heat treatment at temperatures in excess of 500.degree. C. associated with the process convert the material into a high T.sub.C superconductor.
European Patent Application No. EP0288001A (Nissin Electric Company Ltd.) describes a process for producing a superconducting thin film in which a group IIa metal and/or an oxide thereof, a group IIIa metal and/or an oxide thereof, and copper and/or an oxide thereof are supplied on a substrate which is irradiated with a neutral oxygen beam and heat treated at a temperature of 700.degree. C., thereby forming a superconductive thin film of IIa-IIIa-Cu oxide.
Conventional integrated circuits and other semiconductor devices have inherent disadvantages which result from the resistance of the metal tracks that are used as connections between the various devices. The availability of superconducting materials, and in particular those materials which remain superconducting at temperatures above 70.degree. K., has provided an opportunity to reduce this connecting track resistance to zero.
However, due to the structure of the semiconductor of which the metal connecting tracks are commonly located, the formation of superconductor materials on such devices is not readily achieved.
It would therefore be advantageous if a semiconductor device could be provided having a structure which utilises a superconductor material.
European Patent Application No. EP0276746A (Hitachi Limited) describes a superconducting device which has a structure of semiconductor-superconductor or superconductor-normal conductor. The superconductors constituting the superconducting device are made of a superconducting oxide material of K.sub.2 CuF.sub.4 type or perovskite type crystalline structure which contains at least one element selected from Ba, Sr, Ca, Mg and Ra; at least one element selected from La, Y, Ce, Sc, Sm, Eu, Er, Gd, Ho, Yb, Nd, Pt, Lu, and Tc; Cu; and O.
European Patent Application No. EP0295708A (Fujitsu Limited) describes an integrated circuit where the wiring layer is formed of superconductive materials, an electrode formed of normal metals i.e. non-superconductive metals, such as aluminium, connects a part of semiconductor region via a barrier metal such as tin, to the superconductive layer wiring at least at the superconductive layer wiring's side wall which is essentially orthogonal to the layer wiring.